Exergy Analysis of Monoethylene glycol recovery processes for hydrate inhibition in offshore natural gas fields

- Processes for offshore recovery of MEG are assessed in terms of exergy destruction.
- 1st and 2nd Laws of Thermodynamics are combined to obtain exergy flow formulae.
- Reference Environmental Reservoir (RER) is defined via two approaches.
- RER approach #1 sets MEG at air chemical equilibrium: indistinguishable efficiencies.
- RER approach #2 sets MEG ∞ diluted in water + damp air: distinguishable efficiencies.

Gas hydrate formation is an issue in natural gas production. In offshore deepwater scenarios the situation is aggravated by the inaccessibility, salinity, low temperatures and high pressures. Monoethylene glycol (MEG) injection in well-heads is one of the used technologies for flow assurance in gas flowlines. Rich MEG is processed in MEG Recovery Units (MRU) to be recovered as Lean MEG returned to well-heads. In offshore rigs, besides determining energy requirements, the assessment of energy degradation is also important, which can be done by Exergy Analysis. In this work, an exergy formulation is developed and three technologies of offshore MRUs are assessed. Since exergy is a property that depends on the definition of a reference environment reservoir (RER), Exergy Analysis is performed via two RER approaches, both presenting consistent results. Approach #1 prescribes the usual standard atmosphere, with the addend that it is saturated by equilibrium with an infinite body of liquid water and MEG is in chemical equilibrium with air species. Since MEG is spontaneously oxidable, this entails very high exergy flows for MRU streams with MEG. As MEG is practically conserved, high exergy efficiencies result for all MRUs hindering their discrimination. Approach #2 prescribes also the standard atmosphere in equilibrium with liquid water containing MEG at infinite dilution, but not in chemical equilibrium with air. The MEG condition in MRU streams is now more akin with the MEG state in this RER, resulting that MEG streams have lower magnitude of exergy flow, leading to realistic exergy efficiencies that allow MRU discrimination. The underlying reason is that the input exergy flows now have magnitude comparable with exergy losses.

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